The present invention relates to the heating of printed circuit boards, assemblies and the like and more specifically to uniformly heating circuit boards from edge to edge or a desired lateral temperature profile, while being conveyed through a heating area. The present invention also relates to reliably conveying circuit boards, assemblies and the like through heating areas.
In the past, printed circuit boards, assemblies and the like were placed on a flat mesh conveyor belt and conveyed through a series of heating areas or zones. In some cases the heating areas have infrared quartz tubes at very high temperatures. In another instance, flat infrared panel preheaters are provided at more moderate temperatures. The heating devices are used for ceramic substrate firing, for solder reflow of tin/lead electroplate on bare boards, or surface mount soldering on ceramic substrates or conventional board laminates. The term "circuit boards" is used throughout the specification and includes all of the different assemblies described above. In the past, the circuit boards were normally placed by hand or automatically fed to the center of the mesh belt to eliminate the problems of the edges of the belts being either hotter or cooler than the center.
With automation, the flat mesh conveyor belt is replaced by two separate conveyor chains positioned parallel and spaced apart having small pins, tabs or the like to support and convey the circuit boards. The distance between the two conveyors may be varied to suit the size of the circuit boards. This arrangement permits minimum handling of the circuit boards and the machines become integrated to and from each other with the pin/chain conveyors.
Because these conveyors have to be adjustable for different widths, it is generally necessary to pass the rails supporting the conveyors within the heated areas and this can cause problems because the chain conveyors have to extend outside the heating areas and tend to cool. The cold chain conveyors recontact the hot support rails, which can cause the rails to expand unevenly resulting in warping. The conveyors tend to drop the printed circuit boards if the pins on the chain conveyors separate.
In one case efforts have been made to prevent this warping by providing a cooling fluid in the rails to keep them straight and thus prevent them dropping boards. However, this provides a temperature difference between the rails and the circuit board which can result in non uniform temperature profiles across the circuit boards.
In another case attempts have been made to hold the rails straight with heavy clamps. However, adjustments of the rail widths is difficult and time consuming and temperature restabilization has not been successful.
If the rails become too large, and are made of heat conductive materials such as steel or aluminum, then heat is taken by the rails from the quartz heaters or panel heaters resulting in a non uniform temperature profile across the board. This results in a risk of overheating in the center of the boards and/or incomplete solder joints along the edges of the boards where they are cooler because of the presence of the rail and the conveyor.
Attempts have been made to overcome these problems by providing a supplementary heater to heat the side edges of the circuit board. However, by utilizing a supplementary quartz tube heater it was found that not only did it heat the board edge, but also the chain conveyors and the rails. In order to maintain non-distortions in the rail, and a uniform temperature profile of the circuit board it was still necessary to provide cooling within the rail, otherwise uniformity cannot be obtained